The present invention relates to a heat-sensitive transfer medium for use in thermal transfer apparatuses such as thermal printer, facsimile and typewriter. More particularly, it relates to a heat-sensitive transfer medium capable of producing a print image of a high quality on a receiving paper which is poor in surface smoothness.
A heat-sensitive transfer medium which has been widely used heretofore is that wherein a heat-meltable ink layer which is melted in a prescribed temperature is provided on a support, for instance, having a thickness of 3 to 12 .mu.m.
The mechanism of printing using such heat-sensitive transfer medium is as follows: A thermal head is brought into contact with the back surface of the support of the transfer medium. When plural heating elements of the thermal head are selectively activated on the basis of signals for printing to generate heat, portions of the heat-meltable ink layer which are positioned on the heated portions of the support are melted and transferred to a receiving medium, such as plain paper, which is brought into contact with the heat-meltable ink layer, providing transfer images of the heat-meltable ink corresponding to the printing signals on the receiving medium. Thus, the use of the heat-sensitive transfer medium makes possible printing onto a plain paper.
In the case of using the conventional heat-sensitive transfer medium, the transferred ink is bonded to a receiving paper by penetration of the ink in a molten state into the surface layer of the paper. Therefore, the bonding of the ink image is readily subject to the surface property of the receiving paper. For example, when a bond paper having a Bekk smoothness of less than 40 seconds is used as a receiving paper, the bonding of the ink to the paper becomes ununiform, which results in reduction of the image quality.
Further, the use of a conventional heat-sensitive transfer medium having a heat-meltable ink layer wherein a heat-meltable binder having a low melt-viscosity is used causes a blot or blur of a print image. In particular, when the thickness of the ink layer is large, there are problems such as marked blot or blur of the print image and decrease in printing speed. Further, when a print image is formed on a plastic film or sheet for overhead projector (hereinafter referred to as "OHP film"), the ink in a molten state tends to spread laterally so that a sharp image cannot be obtained.
A heat-sensitive transfer medium consisting of a support, a heat-meltable ink layer provided on the support and a transfer-assisting layer provided on the ink layer is proposed (Japanese Unexamined Patent Publication No. 114889/1986). However, a coloring agent contained in the ink layer migrates into the transfer-assisting layer, which results in occurrence of a blot or blur of a print image. Further, a thick ink layer is required in order to obtain a print image with a high density. However, such a thick ink layer is poor in transfer selectivity which means that a portion of the ink layer which is heated with an activated heating element and separated from the support faithfully corresponds to the plane shape of the heating element. The poor transfer selectivity reduces the reproductivity of the same image. Moreover, the thick ink layer ruins the heat conduction so that printing speed is decreased.
On the other hand, a plastic film is usually used as a support for the above-mentioned heat-sensitive transfer medium. However, usual plastic films have a melting or softening temperature of 200.degree. to 300.degree. C. at the highest and also a heat deformation temperature of 100.degree. C. at the highest, while the surface temperature of the thermal head goes up to high temperatures of 300.degree. to 400.degree. C. When such plastic film as the support is heated with the thermal head during printing, the so-called "hot-sticking phenomenon" occurs. The hot-sticking phenomenon involves disadvantages such as sticking of the thermal head to the plastic film (hereinafter referred simply to as "sticking"), which causes hindering in the feeding of the transfer medium; and attaching of some melts (hereinafter referred to as "sticking-dust") of the plastic film to the thermal head.
In order to prevent such hot-sticking phenomenon, heretofore, an attempt that a sticking-preventive layer was provided on the back surface of the plastic film which was to be brought into contact with the thermal head was made. As the sticking-preventive layer, there were proposed a metal layer, a heat-resistant resin layer, a layer composed of benztriazole, an ethyl cellulose layer containing sodium stearyl sulfate and a polyester resin layer containing stearic acid. However, these sticking-preventive layers had drawbacks that when the thickness was small, a sufficient sticking-preventive effect was not attained, and when the thickness was large, the heat-sensitivity was reduced due to an increase in heat capacity and the sticking-preventive layer itself rather causes sticking and sticking-dust.
It is an object of the present invention to provide a heat-sensitive transfer medium capable of producing a transfer image of a very good quality on a receiving medium, particularly a paper having a poor surface smoothness such as bond paper, without being subject to the surface property of the receiving medium, with saving the printing energy at a high speed.
Another object of the present invention is to provide a heat-sensitive transfer medium capable of producing a sharp image on an OHP film.
Still another object of the present invention is to provide a heat-sensitive transfer medium improved in sticking-preventive property as well as the above-mentioned ability of producing transfer images of a high quality.
These and other object of the invetnion will become apparent from the description hereinafter.